I. Field of the Invention
This invention relates generally to spread spectrum communication systems and, more particularly, to signal processing in a cellular telephone communication system.
II. Description of the Related Art
In a wireless telephone communication system, many users communicate over a wireless channel to connect to wireline telephone systems. Communication over the wireless channel can be one of a variety of multiple access techniques which facilitate a large number of users in a limited frequency spectrum. These multiple access techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA). The CDMA technique has many advantages and an exemplary CDMA system is described in U.S. Pat. No. 4,901,307 issued Feb. 13, 1990 to K. Gilhousen et al., entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS," assigned to the assignee of the present invention and incorporated herein by reference.
In the just mentioned patent, a multiple access technique is disclosed where a large number of mobile telephone system users, each having a transceiver, communicate through satellite repeaters or terrestrial base stations using CDMA spread spectrum communication signals. In using CDMA communications, the frequency spectrum can be reused multiple times thus permitting an increase in system user capacity.
The CDMA modulation techniques disclosed in the '307 patent offers many advantages over narrow band modulation techniques used in communication systems using satellite or terrestrial channels. The terrestrial channel poses special problems to any communication system particularly with respect to multipath signals. The use of CDMA techniques permits the special problems of the terrestrial channel to be overcome by mitigating the adverse effect of multipath, e.g. fading, while also exploiting the advantages thereof.
The CDMA techniques as disclosed in the '307 patent contemplate the use of coherent modulation and demodulation for both directions of the link in mobile-satellite communications. Accordingly, disclosed therein is the use of a pilot carrier signal as a coherent phase reference for the satellite-to-mobile unit link and the base station-to-mobile unit link. In the terrestrial cellular environment, however, the severity of multipath fading with the resulting phase disruption of the channel, as well as the power required to transmit a pilot carrier signal from the mobile unit, precludes usage of coherent demodulation techniques for the mobile unit-to-base station link. U.S. Pat. No. 5,103,459 entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM", issued Jun. 25, 1990, assigned to the assignee of the present invention, the disclosure of which is incorporated by this reference, provides a means for overcoming the adverse effects of multipath in the mobile unit-to-base station link by using noncoherent modulation and demodulation techniques.
In a CDMA cellular telephone system, the same frequency band can be used for communication in all base stations. At the base station receiver, separable multipath, such as a line of site path and another one reflecting off of a building, can be diversity combined for enhanced modem performance. The CDMA waveform properties that provide processing gain are also used to discriminate between signals that occupy the same frequency band. Furthermore the high speed pseudonoise (PN) modulation allows many different propagation paths of the same signal to be separated, provided the difference in path delays exceeds the PN chip duration. If a PN chip rate of approximately 1 MHz is employed in a CDMA system, the full spread spectrum processing gain, equal to the ratio of the spread bandwidth to the system data rate, can be employed against paths having delays that differ by more than one microsecond. A one microsecond path delay differential corresponds to differential path distance of approximately 300 meters. The urban environment typically provides differential path delays in excess of one microsecond.
The multipath properties of the terrestrial channel produce at the receiver signals having traveled several distinct propagation paths. One characteristic of a multipath channel is the time spread introduced in a signal that is transmitted through the channel. For example, if an ideal impulse is transmitted over a multipath channel, the received signal appears as a stream of pulses. Another characteristic of the multipath channel is that each path through the channel may cause a different attenuation factor. For example, if an ideal impulse is transmitted over a multipath channel, each pulse of the received stream of pulses generally has a different signal strength than other received pulses. Yet another characteristic of the multipath channel is that each path through the channel may cause a different phase on the signal. For example, if an ideal impulse is transmitted over a multipath channel, each pulse of the received stream of pulses generally has a different phase than other received pulses.
In the mobile radio channel, the multipath is created by reflection of the signal from obstacles in the environment, such as buildings, trees, cars, and people. In general the mobile radio channel is a time varying multipath channel due to the relative motion of the structures that create the multipath. For example, if an ideal impulse is transmitted over the time varying multipath channel, the received stream of pulses would change in time location, attenuation, and phase as a function of the time that the ideal impulse was transmitted.
The multipath characteristic of a channel can result in signal fading. Fading is the result of the phasing characteristics of the multipath channel. A fade occurs when multipath vectors are added destructively, yielding a received signal that is smaller than either individual vector. For example if a sine wave is transmitted through a multipath channel having two paths where the first path has an attenuation factor of X dB, a time delay of .delta. with a phase shift of .THETA. radians, and the second path has an attenuation factor of X dB, a time delay of .delta. with a phase shift of .THETA.+.pi. radians, no signal would be received at the output of the channel.
In narrow band modulation systems such as the analog FM modulation employed by conventional radio telephone systems, the existence of multiple paths in the radio channel results in severe multipath fading. As noted above with a wideband CDMA, however, the different paths may be discriminated in the demodulation process. This discrimination not only greatly reduces the severity of multipath fading but provides an advantage to the CDMA system.
Diversity is one approach for mitigating the deleterious effects of fading. It is therefore desirable that some form of diversity be provided which permits a system to reduce fading. Three major types of diversity exist: time diversity, frequency diversity, and space/path diversity.
Time diversity can best be obtained by the use of repetition, time interleaving, and error correction and detection coding which introduce redundancy. A system comprising the present invention may employ each of these techniques as a form of time diversity.
CDMA by its inherent wideband nature offers a form of frequency diversity by spreading the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth.
Space and path diversity are obtained by providing multiple signal paths through simultaneous links from a mobile unit through two or more base stations and by employing two or more spaced apart antenna elements at a single base station. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing a signal arriving with different propagation delays to be received and processed separately as discussed above. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501 entitled "SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM", issued Mar. 21, 1992 and U.S. Pat. No. 5,109,390 entitled "DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM", issued Apr. 28, 1992, both assigned to the assignee of the present invention.
The deleterious effects of fading can be further controlled to a certain extent in a CDMA system by controlling transmitter power. A system for base station and mobile unit power control is disclosed in U.S. Pat. No. 5,056,109 entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM", issued Oct. 8, 1991, also assigned to the assignee of the present invention.
The CDMA techniques as disclosed in the '307 patent contemplate the use of relatively long PN sequences with each mobile unit user being assigned a different PN sequence. The cross-correlation between different PN sequences and the autocorrelation of a PN sequence, for all time shifts other than zero, both have a nearly zero average value which allows the different user signals to be discriminated upon reception. (Autocorrelation and cross-correlation requires logical "0" take on a value of "1" and logical "1" take on a value of "-1" or a similar mapping in order that a zero average value be obtained.)
However, such PN signals are not orthogonal. Although the cross-correlation essentially averages to zero over the entire sequence length, for a short time interval, such as an information bit time, the cross-correlation is a random variable with a binomial distribution. As such, the signals interfere with each other much the same as they would if they were wide bandwidth Gaussian noise at the same power spectral density. Thus the other user signals, or mutual interference noise, ultimately limits the achievable capacity.
It is well known in the art that a set of n orthogonal binary sequences, each of length n, for n any power of 2 can be constructed, see Digital Communications with Space Applications, S. W. Golomb et al., Prentice-Hall, Inc., 1964, pp. 45-64. In fact, orthogonal binary sequence sets are also known for most lengths which are multiples of four and less than two hundred. One class of such sequences that is easy to generate is called the Walsh function, also known as Hadamard matrices.
A Walsh function of order n can be defined recursively as follows: ##EQU1## where W' denotes the logical complement of W, and W(1)=.vertline.0.vertline..
Thus, ##EQU2##
A Walsh sequence or code is one of the rows of a Walsh function matrix. A Walsh function matrix of order n contains n sequences, each of length n Walsh chips. Each Walsh code has a corresponding Walsh index where the Walsh index refers to the number (1 through n) corresponding to the row in which a Walsh code is found. For example, for n=8 Walsh function matrix given above, the all zeroes row corresponds to Walsh index 1 and the Walsh code 0, 0, 0, 0, 1, 1, 1, 1 corresponds to Walsh index 5.
A Walsh function matrix of order n (as well as other orthogonal functions of length n) has the property that over the interval of n bits, the cross-correlation between all the different sequences within the set is zero. This can be seen by noting that every sequence differs from every other sequence in exactly half of its bits. It should also be noted that there is always one sequence containing all zeroes and that all the other sequences contain half ones and half zeroes. The Walsh symbol which consists all logical zeros instead of half one's and zero's is called the Walsh zero symbol.
On the reverse link channel from the mobile unit to the base station, no pilot signal exists to provide a phase reference. Therefore a method is needed to provide a high-quality link on a fading channel having a low Eb/No (energy per bit/noise power density). Walsh function modulation on the reverse link is a simple method of obtaining 64-ary modulation with coherence over the set of six code symbols mapped into the 64 Walsh codes. The characteristics of the terrestrial channel are such that the rate of change of phase is relatively slow. Therefore, by selecting a Walsh code duration which is short compared to the rate of change of phase on the channel, coherent demodulation over the length of one Walsh code is possible.
On the reverse link channel, the Walsh code is determined by the information being transmitted from the mobile unit. For example a three bit information symbol could be mapped into the eight sequences of W(8) given above. An "unmapping" of the Walsh encoded symbols into an estimate of the original information symbols may be accomplished in the receiver by a Fast Hadamard Transform (FHT). A preferred "unmapping" or selection process produces soft decision data which can be provided to a decoder for maximum likelihood decoding.
An FHT is used to perform the "unmapping" process. An FHT correlates the received sequence with each of the possible Walsh sequences. Selection circuitry is employed to select the most likely correlation value, which is scaled and provided as soft decision data.
A spread spectrum receiver of the diversity or "rake" receiver design comprises multiple data receivers to mitigate the effects of fading. Typically each data receiver is assigned to demodulate a signal which has traveled a different path, either through the use of multiple antennas or due to the multipath properties of the channel. In the demodulation of signals modulated according to an orthogonal signaling scheme, each data receiver correlates the received signal with each of the possible mapping values using an FHT. The FHT outputs of each data receiver are combined and selection circuitry then selects the most likely correlation value based on the largest combined FHT output to produce a demodulated soft decision symbol.
In the system described in the above-referenced '459 patent, the call signal begins as a 9600 bits per second information source which is then converted by a rate 1/3 forward error correction encoder to a 28,800 symbols per second output stream. These symbols are grouped 6 at a time to form 4800 Walsh symbols per second, each Walsh symbol selecting one of sixty-four orthogonal Walsh functions that are sixty-four Walsh chips in duration. The Walsh chips are modulated with a user-specific PN sequence generator. The user-specific PN modulated data is then split into two signals, one of which is modulated with an in-phase (I) channel PN sequence and one of which is modulated with a quadrature (Q) channel PN sequence. Both the I channel modulation and the Q channel modulation provide four PN chips per Walsh chip with a 1.2288 MHz PN spreading rate. The I and the Q modulated data are Offset Quadrature Phase Shift Keying (OQPSK) combined for transmission.
In the CDMA cellular system described in the above-referenced '307 patent, each base station provides coverage to a limited geographic area and links the mobile units in its coverage area through a cellular system switch to the public switched telephone network (PSTN). When a mobile unit moves to the coverage area of a new base station, the routing of that user's call is transferred to the new base station. The base station-to-mobile unit signal transmission path is referred to as the forward link and the mobile unit-to-base station signal transmission path is referred to as the reverse link.
As described above, the PN chip interval defines the minimum separation two paths must have in order to be combined. Before the distinct paths can be demodulated, the relative arrival times (or offsets) of the paths in the received signal must first be determined. The channel element modem performs this function by "searching" through a sequence of potential path offsets and measuring the energy received at each potential path offset. If the energy associated with a potential offset exceeds a certain threshold, a signal demodulation element may be assigned to that offset. The signal present at that path offset can then be summed with the contributions of other demodulation elements at their respective offsets. A method and apparatus of demodulation element assignment based on searcher demodulation element energy levels is disclosed in U.S. Pat. No. 5,490,165, issued Feb. 6, 1996 entitled "DEMODULATION ELEMENT ASSIGNMENT IN A SYSTEM CAPABLE OF RECEIVING MULTIPLE SIGNALS," assigned to the assignee of the present invention. Such a diversity or rake receiver provides for a robust digital link, because all paths have to fade together before the combined signal is degraded.
FIG. 1 shows an exemplary set of signals from a single mobile unit arriving at the base station. The vertical axis represents the power received on a decibel (dB) scale. The horizontal axis represents the delay in the arrival time of a signal due to multipath delays. The axis (not shown) going into the page represents a segment of time. Each signal spike in the common plane of the page has arrived at a common time but was transmitted by the mobile unit at a different time. In a common plane, peaks to the right were transmitted at an earlier time by the mobile unit than peaks to the left. For example the left-most peak spike 2 corresponds to the most recently transmitted signal. Each signal spike 2-7 has traveled a different path and therefore exhibits a different time delay and a different amplitude response. The six different signal spikes represented by spikes 2-7 are representative of a severe multipath environment. Typical urban environments produce fewer usable paths. The noise floor of the system is represented by the peaks and dips having lower energy levels. The task of a searcher element is to identify the delay as measured by the horizontal axis of signal spikes 2-7 for potential demodulation element assignment. The task of the demodulation element is to demodulate a set of the multipath peaks for combination into a single output. It is also the task of the demodulation elements once assigned to a multipath peak to track that peak as it may move in time.
The horizontal axis can also be thought of as having units of PN offset. At any given time, the base station receives a variety of signals from a single mobile unit, each of which has traveled a different path and may have a different delay than the others. The mobile unit's signal is modulated by a PN sequence. A copy of the PN sequence is also generated at the base station. At the base station, each multipath signal is individually demodulated with a PN sequence code aligned to its timing. The horizontal axis coordinates can be thought of as corresponding to the PN sequence code offset which would be used to demodulate a signal at that coordinate.
Note that each of the multipath peaks varies in amplitude as a function of time as shown by the uneven ridge of each multipath peak. In the limited time shown, there are no major changes in the multipath peaks. Over a more extended time range, multipath peaks disappear and new paths are created as time progresses. The peaks can also slide to earlier or later offsets as the path distances change as the mobile unit moves around in the coverage area of the base station. Each demodulation element tracks these small variations in the signal assigned to it.
In a typical wireless telephone communication system, the mobile unit transmitter may employ a vocoding system which encodes voice information in a variable rate format. For example, the data rate may be lowered due to pauses in the voice activity. The lower data rate reduces the level of interference to other users caused by the mobile unit transmissions. At the receiver, or otherwise associated with the receiver, a vocoding system is employed for reconstructing the voice information. In addition to voice information, non-voice information alone or a mixture of the two may be transmitted by the mobile unit.
A vocoder which is suited for application in this environment is described in U.S. Pat. No. 5,414,796, issued May 9, 1995, which is a continuation of U.S. Pat. application Ser. No. 07/713,661 now abandoned, entitled "VARIABLE RATE VOCODER," and assigned to the assignee of the present invention. This vocoder produces from digital samples of the voice information encoded data at four different rates, e.g. approximately 8,000 bits per second (bps), 4,000 bps, 2,000 bps and 1,000 bps, based on voice activity during a 20 millisecond (ms) frame. Each frame of vocoder data is formatted with overhead bits as 9,600 bps, 4,800 bps, 2,400 bps, and 1,200 bps data frames. The highest rate data frame which corresponds to a 9,600 bps frame is referred to as a "full rate" frame; a 4,800 bps data frame is referred to as a "half rate" frame; a 2,400 bps data frame is referred to as a "quarter rate" frame; and a 1,200 bps data frame is referred to as an "eighth rate" frame. In neither the encoding process nor the frame formatting process is rate information included in the data. When the mobile unit transmits data at less than full rate, the duty cycle of the mobile units transmitted signal is the same as the data rate. For example, at quarter rate a signal is transmitted from the mobile unit only one quarter of the time. During the other three quarters time, no signal is transmitted from the mobile unit. The mobile unit includes a data burst randomizer. Given the data rate of the signal to be transmitted, the data burst randomizer determines during which time slots the mobile unit transmits and during which time slots it does not. Further details on the data burst randomizer are described in copending U.S. patent application Ser. No. 08/194,823 filed Feb. 14, 1994, which is a continuation of U.S. patent application Ser. No. 07/846,312, now abandoned, entitled "DATA BURST RANDOMIZER," filed and assigned to the assignee of the present invention.
At the base station, each individual mobile unit signal must be identified from the ensemble of call signals received to be demodulated back into the original call signal of the mobile unit. A system and method for demodulating a mobile unit signal received at a base station is described, for example, in the '459 patent. FIG. 2 is a block diagram of the base station equipment described in the '459 patent for demodulating a reverse link mobile unit signal.
A typical prior art base station comprises multiple independent searcher and demodulation elements. The searcher and demodulation elements are controlled by a microprocessor. In this exemplary embodiment, to maintain a high system capacity, each mobile unit in the system does not transmit a pilot signal. The lack of a pilot signal on the reverse link increases the time needed to conduct a survey of all possible time offsets at which a mobile unit signal may be received. Typically a pilot signal is transmitted at a higher power than the traffic bearing signals thus increasing the signal to noise ratio of the received pilot as compared to the received traffic channel signals. In contrast, ideally each mobile trait transmits a reverse link signal which arrives with a signal level equal to the power level received from every other mobile unit therefore having a low signal to noise ratio. Also, a pilot channel transmits a known sequence of data. Without the pilot signal, the searching process must examine all possibilities of what data may have been transmitted.
FIG. 2 shows an exemplary embodiment of a prior art base station. The base station of FIG. 2 has one or more antennas 12 receiving CDMA reverse link mobile unit signals 14. Typically, an urban base station's coverage area is split into three sub-regions called sectors. With two antennas per sector, a typical base station has a total of six receive antennas. The received signals are down-converted to baseband by an analog receiver 16 that quantizes the received signal I and Q channels and sends these digital values over signal lines 18 to channel element modem 20. A typical base station comprises multiple channel element modem like channel element modem 20 (not shown in FIG. 2). Each channel element modem 20 supports a single user. In the preferred embodiment, channel element modem 20 comprises four demodulation elements 22 and eight searchers 26. Microprocessor 34 controls the operation of demodulation elements 22 and searchers 26. The user PN code in each demodulation element 22 and searcher 26 is set to that of the mobile unit assigned to that channel element modem 20. Microprocessor 34 steps searchers 26 through a set of offsets, called a search window, that is likely to contain multipath signal peaks suitable for assignment of demodulation elements 22. For each offset, searcher 26 reports the energy it finds at that offset to microprocessor 34. Demodulation elements 22 are then assigned by microprocessor 34 to the paths identified by searcher 26. Once one of demodulation elements 22 has locked onto the signal at its assigned offset, it then tracks that path on its own without supervision from microprocessor 34, until the path fades away or until it is assigned to a new path by microprocessor 34.
Each of demodulation elements 22 is highly similar in structure to the others. FIG. 3 illustrates in further detail demodulation element 22 of FIG. 2. In FIG. 3, the digitized input signal is assumed to be a Quadrature Phase Shift Keyed (QPSK) signal having in-phase (I) and quadrature-phase (Q) signal samples. The I and Q signal samples, each being a multiple-bit value, are input to decimator and despreader 46. Typically the I and Q signal samples are over-sampled in that the input is received at a data rate higher than the chip rate. In decimator and despreader 46, the data is decimated from the over-sampled data rate to the PN chip rate. The data is then despread by decimator and despreader 46 using the same PN sequence used to modulate this signal in the mobile unit.
Decimator and despreader 46 outputs despread I and Q signal components to the accumulator 48. Accumulator 48 accumulates the despread I and Q signal components over a Walsh Chip period to produce accumulated I and Q chip data. The accumulated I and Q chip data is then processed by the Fast Hadamard Transformer (FHT) and selector 52. The FHT portion of FHT and selector 52 correlates the accumulated I and Q chip data with all possible Walsh sequences. Each I and Q correlation result is then used to estimate the magnitude of the corresponding Walsh symbol. The magnitude estimates from each of the I and Q correlation results are compared with each other. The Walsh symbol corresponding to the I and Q correlation result that has the biggest magnitude is selected by the selector portion of FHT and selector 52 as the demodulated Walsh symbol. The demodulated Walsh symbol is output along with the corresponding estimated magnitude of that Walsh symbol.
Due to the different arrival times of signal paths assigned to the different demodulation elements, demodulation element 22 also performs a de-skewing of the symbols. The output to combiner 28 (FIG. 2) must be delayed by deskew buffer 54 such that each demodulation element 22 provides data synchronized in relation to the other demodulation elements.
Energy accumulator and lock detector 44 sums a series of consecutive Walsh symbol magnitudes. The resulting sum is output to microprocessor 34 (FIG. 2). The resulting sum is also compared with a threshold to indicate a locked or unlocked state by energy accumulator and lock detector 44.
The arrival time of the signal path assigned to demodulation element 22 may change over time because of movement of the mobile unit or change in the mobile unit's surroundings. Therefore demodulation element 22 incorporates time tracking circuitry. Decimator and despreader 46 outputs an early and a late version of despread I and Q signal components to be used in the time tracking process. Accumulator 50 accumulates the early and the late, despread I and Q signal components over a Walsh chip period to produce accumulated early and late, I and Q chip data. Delay 40 delays the output of accumulator 50 until the results of FHT and selector 52 are available. Note that delay 40 must store large blocks of data and requires a large die area in an integrated circuit chip implementation. Early and late metric generation 38 then multiplies the accumulated early and late, I and Q chip data by the Walsh sequence corresponding to the demodulated Walsh symbol and accumulates the result to find the magnitude of early and late, I and Q Walsh symbols. The early symbol magnitude is subtracted from the late symbol magnitude to produce an error metric. The error metric is output to time tracking 36. Time tracking 36 uses the error metric to determine if the despreading operation in decimator and despreader 46 is running ahead, behind, or on time. Microprocessor 34 keeps track of absolute demodulation path time of the demodulation element 22.
Referring again to FIG. 2, the output of FHT and selector 52 for each demodulation element 22 is then combined with those of other demodulation elements by combiner 28. The output of combiner 28 is a "soft decision" demodulated symbol, weighted by the confidence that it correctly identifies the originally transmitted Walsh symbol. The soft decision is then passed to forward error correction decoder 29 for further processing to recover the original call signal. This call signal is then sent through digital link 30, such as a T1 or E1 link, that routes the call to public switched telephone network (PSTN) 32.
Like each demodulation element 22, each searcher 26 contains a demodulation data path with an FHT processor capable of performing one FHT transform during a time period equal to the period of a Walsh symbol. The FHT processor is slaved to "real time" in the sense that during every Walsh chip interval one value is input to the FHT and one value is output from the FHT. To provide a rapid searching process, more than one searcher element must be used. Searcher elements 26 continually scan in search of a particular mobile unit's information signal as controlled by microprocessor 34. For each offset processed, each searcher 26 finds the correlation energy at that offset by despreading the antenna samples, accumulating them into Walsh chips that are input to the FHT transform, performing the FHT transform and summing the maximum FHT output energy for each of the Walsh symbols for which the searcher dwells at an offset. The final sum is reported back to microprocessor 34.
The multipath environment is constantly changing as the mobile unit and other reflective objects move about in the base station coverage area. The number of demodulation elements required is a function of the number of paths generally found to be usable at any point in time. To meet these needs, the FIG. 2 system has two searchers 26 and one demodulation element 24 for each of four demodulator integrated circuits (IC's) used, for a total of four demodulation elements and eight searchers per channel element modem. Each of these twelve processing elements contains a complete demodulation data path, including the FHT processor which takes a relatively large, costly amount of area to implement on an integrated circuit. In addition to the four demodulator IC's the channel element modem also has a modulator IC and a forward error correction decoder IC for a total of 6 IC chips. A powerful and expensive microprocessor is needed to manage and coordinate the demodulation elements and the searchers. As implemented in the modem of FIG. 2, these circuits were completely independent and require the close guidance of microprocessor 34 to sequence through the correct offsets, and handle the FHT outputs. Every Walsh symbol microprocessor 34 receives an interrupt to process the FHT outputs. This interrupt rate alone necessitates a high powered microprocessor.
It would be advantageous if the six IC's required for a modem could be reduced to a single IC needing less microprocessor support, thereby reducing the direct IC cost and board-level production cost of the modem, and allowing migration to a lower cost microprocessor (or alternately a single high powered microprocessor supporting several channel element modems at once). Just relying on shrinking feature sizes of the IC fabrication process and placing the six chips together on a single die is not enough. The fundamental architecture of the demodulator needs to be redesigned for a truly cost effective single chip modem. From the discussion above, it should be apparent that there is a need for a signal receiving and processing apparatus that can demodulate a spread spectrum call signal in a lower cost and more architecturally efficient manner.